1. Field of the Invention
The present invention relates to new, water soluble polyisocyanate addition products, to a process for their preparation and to their use as dispersing agents for incorporating solids into aqueous lacquers or coating compositions.
2. Description of the Prior Art
High mechanical forces are necessary to incorporate solids into liquid media. This depends to a large extent on the wettability of the solid by the surrounding medium and on its affinity for this medium. In order to reduce these dispersion forces, it is customary to employ dispersing agents which facilitate incorporation. These are primarily surface-active agents (surfactants) having an anionic, cationic or nonionic structure. These substances are either deposited directly on the solid or are added to the dispersing medium, in relatively small amounts in each case. The dispersion energy is significantly reduced by these surfactants.
It is also known that these solids tend to agglomerate again after dispersion, which nullifies the dispersion energy expended previously and results in serious problems. This phenomenon is explained by London/van der Waals forces, which relate to the mutual attraction between the solids. In order to overcome these forces of attraction, absorption layers have to be deposited on the solids, e.g., by using surfactants.
During and after dispersion an interaction occurs between the surrounding medium and the solid particle, which results in the desorption of the surfactant and its replacement by the surrounding medium, which is present in a higher concentration. However, in most cases the surrounding medium is not capable of forming stable absorption layers and the whole system breaks down. This is manifested, for example, by an increase in the viscosity of liquid systems and by loss of gloss and color shifts in lacquers and coatings.
There have been many efforts aimed at providing effective dispersants for solids, particularly for pigments, which facilitate the incorporation of solids into solvent-based coating compositions and enable coating compositions to be produce are stable to sedimentation. Dispersing agents of this type are described in EP-A 0,154,678, EP-A 0,205,510 and EP-A 0,335,197, for example.
The dispersing agents described in these publications are based on polyisocyanates having a functionality of .gtoreq.2.5. Due to the high functionality, most of these polyisocyanates are highly viscous and can only be produced and made available as dilute solutions containing considerable amounts of organic solvents. These solvents can have a disadvantageous effect on the final product, particularly when the latter is a dispersing agent for aqueous coating compositions. Therefore, the solvents have to be removed by distillation after the production of the dispersing agent, which causes longer production times and higher costs.
It is known in the art that dispersing agents based on polyisocyanates having a functionality of .ltoreq.2.5 do not exhibit the desired high efficiency of the dispersing agents described above.
An object of the present invention is to provide a new process for producing highly effective dispersing agents in which low viscosity polyisocyanates are used and organic solvents are not necessary. It is an additional object of the present invention for the dispersing agents to be suitable for dispersing solids, particularly pigments, into aqueous coating compositions.
These objects may be achieved with the dispersing agents according to the invention which are described below and obtained by the process according to the invention.